The present invention relates to a steering apparatus for an automotive vehicle, which has an electric drive unit such as an electric motor to vary a steering ratio, or steering gain, (an angle ratio between a steering wheel angle and a road wheel angle) of the vehicle. The present invention also relates to a steering ratio, or steering gain, control method for an automotive vehicle.
Japanese Laid-Open Patent Publication No. 2003-182622 proposes a steering apparatus for an automotive vehicle that is equipped with an electric motor to vary a steering ratio of the vehicle according to a steering ratio characteristic. There is a possibility that the electric motor receives a large electric current and gets overheated e.g. when the steering wheel is held or turned repeatedly to a steering lock position. In order to prevent such motor overheating, the above-proposed steering apparatus adjusts a steering gain R of the steering ratio characteristic in response to a temperature increase in the motor in such a manner that the steering gain comes closer to its base level Rc (at which the steering gain R stands under no steering ratio control) as shown in FIG. 6.